This invention relates to an electrical high voltage bushing assembly which is particularly suitable for connecting metal enclosed high voltage devices to a high voltage circuit in a power distribution system.
In power distribution systems, electrical connection is often made to oil insulated devices, such as reclosers, transformers, circuit breakers, and the like. Typically, the devices are enclosed in a metal clad enclosure which is maintained at a lower voltage than the distribution voltage. Often the enclosure is grounded. In these circumstances, the distribution voltage must be supplied to the device in the interior of a grounded enclosure while avoiding electrical breakdown between the distribution circuit and the grounded enclosures. Such breakdown of the insulating system may involve unacceptably high currents and cause a power outrage. The power connection is normally made through a high voltage electrical bushing assembly. Generally, a conventional feed-through bushing assembly for such high voltage applications includes an elongated tubular housing or shell formed of porcelain. The feed-through bushing intrudes to the inside of the enclosure to connect, or feed, the contained device distribution voltage. A relatively large conductor is coaxially mounted within the porcelain, with the opposite ends of the conductor connected to suitable contact members which are sealed to the ends of the shell by suitable sealing means. The conductor is spaced from the walls of the shell and the annular space surrounding the conductor is partially filled with a suitable high strength dielectric fluid, with a small volume providing for expansion of the fluid under operating conditions being also incorporated. The dielectric fluid is usually an insulating oil. The bushing structure should also avoid high voltages stresses to the extent possible, and where high voltage stresses cannot be avoided, should localize them to areas where their adverse effects can be minimized. Obviously, when one is seeking to connect a device within a grounded structure to distribution line voltage, relatively large differences in voltage exist. Voltage stress is proportional to voltage difference and inversely proportional to the distance between locations at differing voltages. High levels of voltage stress may lead to flashover along the interface between solid and fluid insulators or the puncture of solid or fluid insulating materials, causing electrical fault. High voltage stress may result in a corona. Corona is low current level conduction in the insulating media. Corona discharges are often accompanied by objectional levels of radio interference. The partial breakdown of the insulating media by corona may be followed by ionization and arcing, and may result in high fault currents. Corona in oil will degrade the oil. In order to prevent creation of high voltage stresses, the shell is provided with conductive coatings on the interior and exterior surfaces. Multiple conductive surfaces are used in capacitively graded bushings and may be used to provide a capacitive voltage tap for monitoring and test purposes. In addition, an insulating tube may be disposed within the annular chamber to increase dielectric strength while positively spacing the shell and the center conductor. The insulating tube may incorporate a conductive coating or a tubular conductive shell. Terminal sealing means includes an inner stepped structure with internal clamping of the conductor to the porcelain shell. The lower terminal sealing means consists of a pair of elastomer gaskets which seal between the center conductor and the interior wall of the shell.
The exterior of the bushing shell includes a centrally located annular mounting flange adapted to be clamped in sealing engagement against the device housing, with a suitable insulating gasket to seal the connection. A soft aluminum ring is interposed between the clamp mechanism and the flange. The flange is provided with a conductive coating which extends over the flange portion adjacent the metal clad housing and continues over a substantial portion of the outer surface of the shell located within the metal clad housing. The aluminum ring and clamp unit connects the conductive coating to the device housing to place the coating at a corresponding ground or reference potential. Alternately, where a conductive coating has not been extended to a metallic clamping unit, "U"-shaped conductive clips surrounding the insulating gasket have been used to ground the coating.
The inner end of the bushing disposed within the protective oil is preferably provided with an exterior corona shield, which provides improved electrical potential distribution near the lower end of the bushing. The shield includes a tubular portion which is positioned adjacent to the inner end portion of the conductive coating and a conical portion flaring outwardly and opening towards the end of the conductor connected to the device. The tubular portion is formed of a suitable conductive metal and incorporates a clamping end which is provided with a plurality of edge slots or slits to permit a compression electrical connection with the conductive coating on the housing in conjunction with a clamp. An inner metallic "garter" type coil spring is located between the shell and the conical portion to position the shield and provide a proper centering of the corona shield. The metallic spring is typically formed of beryllium copper and in part provides some electrical grading at the end of the conductive coating. Unfortunately, the metallic spring is a good conductor and functions as an electrode. This spring located in a circumferential groove in the shell creates an undesirable voltage stress across the remaining wall of the shell.
Further, with the very tight clamping of the corona shield in position as normally required, a chamber is defined between the bushing and the corona shield. Care must be taken that air and other gases within the oil not be trapped within such chamber. Air and gases will, of course, change the voltage distribution in the corresponding region, and may stress the porcelain beyond its puncture point.
Although the bushing structure has been found to provide satisfactory application in high voltage environments, the bushing is relatively expensive and has certain disadvantages from a standpoint of maintaining a fully effective bushing construction over long periods of time. For example, application of a conductive coating on the inner bore of the porcelain bushing is expensive and difficult to apply consistently. When silver is used to coat the exterior outer surface near the flange of the shell, an intermediate coating should be applied to provide additional electric field grading at the termination of the silver coating. Further, such metalic coatings must be protected from chemical and mechanical damage by an additional coating. Heat shrink tubing has proved to be a suitable additional coating in many applications. Unfortunately, even heat shrink tubing will not prevent flaking of the silver in all applications. Further, the silver coating and aluminum mounting ring form a joint which is an electrolytic cell and is therefore subject to the corrosion of the members. Unchecked corrosion will eventually cause destruction of the conducting path from the bulk of the silver coating to the device housing. A significant reduction in the conducting path creates a condition which is prone to generation of both external and internal electric discharges. Such a condition, of course, must be avoided to maintain an effective bushing.
The upper region of the stepped bushing with the restraining of the conducting rod at such point creates high pressure points tending to produce chipping of the procelain. The chipping often results in a loosening of the desired fluid-tight connection of the conducting rod and interconnecting top terminal interface and may allow oil to leak out as well as permit the leakage of moisture into the bushing. Any introduction of moisture into the oil significantly reduces the dielectric strength of the oil and thereby the effectiveness of the high voltage bushing.
Thus, although the high voltage bushing heretofore used provided effective usage in electrical feedthrough type bushings, there is a need for a more effective and a less expensive bushing which can be used in a wide range of applications for connecting switch gear and other high voltage devices to power distribution systems.